Hitherto, as a method for transporting live fish and the like, there has been mainly employed such a method that they are put in a water tank having a large volume and are transported with the entire tank. However, this method requires a high transport cost and, in order to reduce the cost, various methods have been tried. For example, lots of fish are transported in sea water or fresh water with cooling by addition of ice or the like to lower movements of fish and to make the range of their movements small. Or, several bamboo baskets each of which contains one fish are piled up and the entire baskets are placed in sea water or fresh water to limit movements of fish.
Further, in Japanese Patent Kokai No. 51-103000, there is described a method for transporting fish wherein fish are anesthetized with an anesthetic and cooled in a water tank, and then transferred into another water tank containing no anesthetic. In "Possibility of Applying Anesthesia by Carbon Dioxide in the Transportation of Live Fish", Bull. Jap. Soc. Sci. Fish, 49 (5), 725-731 (1983), there is described a method using NaHCO.sub.3 and an acid as well as a method employing aeration with CO.sub.2 and O.sub.2.
On the other hand, in the selection of fry, there has been known a method wherein fry are anesthetized by bubbling CO.sub.2 gas through sea water to lower pH. Among fry, there are malformed fry having no or deformed bladders by nature, which frequently results in shape abnormality after grown up. Accordingly, they should be thinned out at the initial stage of cultivation. However, there is no difference between the ordinary state of movements of malformed fry and that of normal fry, and it is very difficult to differentiate such malformed fry from normal fry by appearance. Provided that, when fry are anesthetized, there is observed such a phenomenon that normal fry rise to the surface with facing their bladders upward, while malformed fry sink below the water. Thereby, whether fry are normal or abnormal can be readily judged, and the selection of fry can be readily conducted. In this known method, CO.sub.2 is bubbled into sea water to lower pH to the critical value (usually 5.6) or lower to anesthetize fish forcibly.
In this conventional aeration with CO.sub.2, it is known that, when pH drops to a certain critical value, fish show vigorous swimming motions just before they are put under anesthesia, although it is varied according to kinds of fish. This is known as a so-called "exaltation phase at initial stage under anesthesia". As described in the above Bull. Jap. Sci. Fish, 49 (5), in general, when a drug is effected on fish, the following stages are usually observed in order according to the degree of intensity of its activity.
(1) Fish show vigorous swimming motions with so-called "surfacing" like behavior.
(2) Fish response to stimulation becomes very weak but fish keep their balance. Respiration movements are deep and hard.
(3) Fish lose their balance and lie on their sides to almost repose. Respiration movements are deep and hard.
(4) Fish lie on their sides in repose. Respiration movements are light and hard.
(5) Fish lie on their sides in repose. Respiration movements are irregular.
(6) Fish lie on their sides in repose. Respiration movements cease to die.
These have been also confirmed by the present inventors' experiments and, when fish show vigorous swimming motions, sometimes, they jump up about 30 cm or more above the water and often damage themselves by hitting against the edge or wall of a water tank. It is clear that this decreases in value of live fish, and therefore, this stage is a problem in anesthesia of fish. However, it is considered that this stage is unavoidable in a method of anesthetizing fish by aeration with CO.sub.2.
When transporting live fish, it is necessary to maintain fish in the state of the above (3) and, in addition, it is desirable to directly introduce fish to the above state (3) without passing through the above states (1) and (2). Accordingly, in transporting live fish, the above various methods have been tried. However, they are still insufficient because, in the method using an anesthetic, it is required to keep a water temperature low or, in CO.sub.2 aeration, the above damage of fish is unavoidable. Therefore, the above methods are not widely employed in practice. Particularly, in the case of very valuable high grade marketable fish such as red sea bream or aquarium fish such as colored carp, CO.sub.2 aeration can not be employed in practice because, as described above, fish are liable to be damaged by this method and damage in these kinds of fish remarkably reduces their value. Therefore, it has been requested in this field to develop a method for anesthetizing fish without damage thereof.
Besides fish, the similar problems are present in transporting other aquatic animals such as squids, cuttlefish, octopuses, prawns, lobsters, crabs and other valuable high grade aquatic animals because there is a remarkable difference in commercial values between transporting them with keeping alive and transporting them without keeping alive such as frozen products. Therefore, it is desirable to transport them with keeping alive. Particularly, when squids or octopuses are transported by putting them in a water tank, there is such a problem that they hit their heads against the wall of the tank to die, or show vigorous swimming motions and eject ink to contaminate water. This requires cleaning means such as filtration or adsorption means. Thereby, an additional cost is required and, further, a yield is lowered.